what is the deal with my 225 /six compression?

[wjajr]

Compression test can be a tool to fine tune valve lash.

Set lash to: Intake @ 0.010”and Exhaust @ 0.012” using previously described method hot and running. Drive the car for several hundred miles varying rpm as if you were braking in a new engine. Hopefully this will free up any stuck rings, and help reseat rings if car has been sitting for a stretch of time. Then perform a second compression test, and observe any pressure differences from first test.

If you find that any cylinder pressures are 10% or more lower than adjacent cylinders, loosen that exhaust valve in 0.002” increments ( 0.014”) retest that cylinder to see if pressure increased. Engine will be a bit more clattery, but should run smoother at idle. What you are doing is making exhaust valve open later and close sooner allowing pressure to increase in that cylinder, in other words decreasing valve event overlap.

Camshaft lobe wear, valve recession and other variables in a worn engine will cause pressure differences as will ring and valve problems. My 225 has a big mystery cam with low vacuum at in gear idle because of a lot of overlap and with head mods making 9.5:1 cr. Engine always had uneven exhaust pulse when listening at rear bumper, and causing me to think there was a ring problem in one hole.

I have had to experiment to find an ideal lash setting, and discovered setting lash by compression readings, all cylinders were made the same within 2 psi at 163 to 165 psi. This smoothed out uneven exhaust pulse, increased & smoothed out in gear idle vacuum from a wagging needle between 1”- 6” Hg to a just a 2” sweep between 4”- 6” Hg. which 390 Holley 4v likes a lot better. I ended up with same setting for intake and exhaust, some cylinders at 0.024” ranging up to 0.028” on one cylinder that was at 140 psi.

 

[Bill Dedman]

Compression test can be a tool to fine tune valve lash.

Set lash to: Intake @ 0.010”and Exhaust @ 0.012” using previously described method hot and running. Drive the car for several hundred miles varying rpm as if you were braking in a new engine. Hopefully this will free up any stuck rings, and help reseat rings if car has been sitting for a stretch of time. Then perform a second compression test, and observe any pressure differences from first test.

If you find that any cylinder pressures are 10% or more lower than adjacent cylinders, loosen that exhaust valve in 0.002” increments ( 0.014”) retest that cylinder to see if pressure increased. Engine will be a bit more clattery, but should run smoother at idle. What you are doing is making exhaust valve open later and close sooner allowing pressure to increase in that cylinder, in other words decreasing valve event overlap.

Camshaft lobe wear, valve recession and other variables in a worn engine will cause pressure differences as will ring and valve problems. My 225 has a big mystery cam with low vacuum at in gear idle because of a lot of overlap and with head mods making 9.5:1 cr. Engine always had uneven exhaust pulse when listening at rear bumper, and causing me to think there was a ring problem in one hole.

I have had to experiment to find an ideal lash setting, and discovered setting lash by compression readings, all cylinders were made the same within 2 psi at 163 to 165 psi. This smoothed out uneven exhaust pulse, increased & smoothed out in gear idle vacuum from a wagging needle between 1”- 6” Hg to a just a 2” sweep between 4”- 6” Hg. which 390 Holley 4v likes a lot better. I ended up with same setting for intake and exhaust, some cylinders at 0.024” ranging up to 0.028” on one cylinder that was at 140 psi.

RE: "some cylinders at 0.024” ranging up to 0.028” on one cylinder that was at 140 psi."

Solid lifter camshafts are ground with clearance ramps, the purpose of which is to gently and slowly close up the space we call "valve clearance" so that when the unit loading goes up on that lifter/camshaft interface, (when the valve opens,) the cam and lifter will already be touching, and there will be no shock-loads imparted to that junction of parts, when push comes to shove.

If you open up the clearance on that valve-train to a value that is far greater than what the design parameters are set to "cover" (say, .010" for instance,) it seems to me that the profile of the cam will be in a more-aggressive lift-mode at that point, (when they DO finally come together,) and will "hammer" the lifter with a force it was never designed to handle. The carefully-designed clearance ramp in no longer there, once the space between the lifter and the cam is increased to double what it was intended to be.

I don't want to think about the consequences of ongoing operation with this scenario...

Does that make sense, and, if it seems not to, please explain what reasoning is faulty, here.... I may be missing something, and don't know it.ops:

 

[wjajr]

This particular cam has very long ramps, at 0.006” the valve is open for 315 degrees, and closed for only about 20 degrees per degree wheel at zero lift. I had been advised multiple times by some of the slant racers over on /6dotorg to loosen lash, and never went past 0.018” maintaining a difference between intake and exhaust settings; having relashed the engine 10 to 15 times in the last 12,000 miles. These attempts yielded mostly poor results, and difficult carburetor tuning particularly at idle due to a poor vacuum signal aggravating power valve impute, and idle mixture control monitored via an o2 sensor.

I than looked for cam cards listed on /6 dot org of cams close to my cam, and most solid lifter cams of this lift and duration use lash adjustments in the 0.024” range; so I gave it a shot. With a little lash fiddling, and compression monitoring as able to tune my engine.

All this about my situation is just an example of how much lash adjustments can effect change in engine’s tune.

I used my experience as an example of how to manipulate cylinder pressure, and only recommended a 0.002” or so loosening of exhaust valve to see if it would correct for some difference in tolerances between lobes and valve train to even out cylinder pressures.

With a stock cam one has a starting place; 0.010” & 0.012”. Some of the /6 veterans clime they can listen to exhaust note and tapping of tappets and I believe them, tune a slant to be very smooth and responsive in a stock engine. To the inexperienced ear this is difficult, well for me anyway with a pronounced lopping engine, but compression tester enables one to achieve same results.

 

[Bill Dedman]

This particular cam has very long ramps, at 0.006” the valve is open for 315 degrees, and closed for only about 20 degrees per degree wheel at zero lift. I had been advised multiple times by some of the slant racers over on /6dotorg to loosen lash, and never went past 0.018” maintaining a difference between intake and exhaust settings; having relashed the engine 10 to 15 times in the last 12,000 miles. These attempts yielded mostly poor results, and difficult carburetor tuning particularly at idle due to a poor vacuum signal aggravating power valve impute, and idle mixture control monitored via an o2 sensor.

I than looked for cam cards listed on /6 dot org of cams close to my cam, and most solid lifter cams of this lift and duration use lash adjustments in the 0.024” range; so I gave it a shot. With a little lash fiddling, and compression monitoring as able to tune my engine.

All this about my situation is just an example of how much lash adjustments can effect change in engine’s tune.

I used my experience as an example of how to manipulate cylinder pressure, and only recommended a 0.002” or so loosening of exhaust valve to see if it would correct for some difference in tolerances between lobes and valve train to even out cylinder pressures.

With a stock cam one has a starting place; 0.010” & 0.012”. Some of the /6 veterans clime they can listen to exhaust note and tapping of tappets and I believe them, tune a slant to be very smooth and responsive in a stock engine. To the inexperienced ear this is difficult, well for me anyway with a pronounced lopping engine, but compression tester enables one to achieve same results.

Thanks for that explanation. Your comment in a previous note ("I ended up with same setting for intake and exhaust, some cylinders at 0.024” ranging up to 0.028” on one cylinder that was at 140 psi.") made me wonder about possible damage to the lobe due to missing the clearance ramp completely, and encountering a too-aggressive opening rate, once the wider clearance was closed up.

That was my only concern...

 

[/6 Matt]

Right before you posted wjajr, I was actually wondering about the possibility of tuning the engine and correcting for cylinder pressures and loading by valve adjustment. I got to say you do have a good idea, but honestly I would set it to what cam specifications are because even though it might not do immediate damage, it could do slow long term damage for the reasons Bill pointed out.

I'm thinking, bent pushrods and rockers with a hole punched in them by the pushrods or valves for that matter. I've seen the rocker thing happen.

On another note, I've noticed we now have a few different people stating different methods for adjusting the lash. This is a constant debate among slanters, in the thread I posted from SS.org they even debate about it there too. TO THE ORIGINAL POSTER: Pick whichever method you like better and do it. Or try both and see which you like better and/or which yeilds better results.

 

[/6 Matt]

Remember, everyone scratches their crack a different way, but at the end of the day we still have our fingers in our butt cracks.

Words of wisdom from a teenager...

 

[wjajr]

Personally I prefer to adjust lash when engine is running, however with big overlapping cam, and higher than factory idle, engine is moving too much, oil squirts all over inner fender, firewall, floor, and windshield; one PIA. Until I added more mufflers I could not hear valve to rocker sounds during lash adjustment. So now I warm her up real good, shut her down, pull all plugs, valve cover, and adjust all of them in pairs; 1&6 etc. with three rotations of crank; it’s quick, much cleaner, and wrist & knuckles feel better after.

Factory Service Manual directs one to adjust while engine is fully warmed up, and running. The important point is to have engine up to operating temperature to take into account all the liner thermal expansion of its moving parts of valve train. Lashing a cold engine without adding this heat expansion into the adjustment can cause too tight lash setting once engine is up to temperature, and burnt valves from never fully closing.

Lash too tight is much more of a problem than too loose. Clattering won’t hurt anything within reason. You can hear the difference in sound on a quiet engine/exhaust; as lash is loosened going from sowing machine tic-tic-tic to tap-tap-tap to clat, clat, clat. Clat, clat is where things begin to get too loose.

As far as force exerted on rocker and valve stem, spring load is the same when contact is initially made a wee bit further along opening ramp, stem velocity is a bit higher and accelerates the further along the ramp contact with rocker is made. I haven’t read about any harm in running with 0.028” lash, some of the HP /6 engines run this kind of lash in excess of 6000 rpm., just make for a noisy valve train. If using a stock cam, one will get a sluggish engine at these big lash settings because valves are not fully opening, or staying open long enough for a complete A/F cylinder fill.

Once again loosening lash by increments of 0.002” to say 0.004”on a stock engine will make huge changes.

 

[/6 Matt]

Even though the factory service manual says that, there are so many people that do it differently because the beauty of this hobby is you can do things your own way.

If it works for you wjajr, awesome!!! It might work for me too but it still makes me nervous to think about it lol!

 

[wjajr]

Bill:

Thanks for that explanation. Your comment in a previous note ("I ended up with same setting for intake and exhaust, some cylinders at 0.024” ranging up to 0.028” on one cylinder that was at 140 psi.") made me wonder about possible damage to the lobe due to missing the clearance ramp completely, and encountering a too-aggressive opening rate, once the wider clearance was closed up.

That was my only concern...

At 0.006 lift lifter crank has moved 20 degrees, at 0.010” 30 degrees, and 0.030” 50 degrees. By shortening ramp or increasing rate of lift I have shortened the overlap event that has been causing low vacuum and torque at low rpm. I have moved the torque curve down the rpm band a little, and clipped some of the high rpm breathing which makes engine a more drivable on street. This engine begins to pull at about the same point a stock slant gives up at 4000 rpm, and pulls hard to 6000 rpm. Last 1/8th run was 10.3 seconds @ 68.3 mph with 3.55:1 rear gear, this is where a stock slant is running a quarter mile off the show room floor.

I think and have been told I need more compression for this cam to perform. Calculations indicate 10.5:1 static would yield dynamic ratio around 8.1:1 which would allow tightening of lash to take advantage of long ramps and better cylinder fill on mid-grade pump gas. But, this is all way beyond scope of this thread, and I apologize for hijacking it.

Additional technical engine chatter on this topic can be found on /6 dotorg if anyone is interested in this stuff.

 

[Ruger64]

Oh crap!! I know this is an old thread.
However, I came across it and realized I did it wrong?
I did a compression test, since I have a slight rough idle, and a slight pop out of the tailpipe once in a while, and some black stuff shooting out the tailpipe when starting. It does run a bit rich, but not as much since the rebuilt.
I think my carb, even though it was just rebuilt, is wore out. Carter BBS. It is even leaking gas ever so slightly out of the top gasket. It's damp there. Maybe old warped carb.
Although, she does run like a beast! 1965/225

I didn't remove all the plugs??? And didn't hold the throttle wide open during cranking??
I removed one at a time, and put them back.
I didn't know they all had to be out of the engine at the same time? Does it matter that much?
My readings were:
1-125
2-124
3-125
4-125
5-125
6-125
Should I redo it with the plugs all out?

 

[krazykuda]

Should I redo it with the plugs all out?

If you want to, you can. It looks ok from what you did, re-doing it will give you better numbers and better accuracy.


It's good to get a snap shot of what your engine is over time. If you re-do the test, do it two ways, wet and dry.

By dry, you test each cylinder as is after removing the spark plugs.

The wet test you get an oil squirt can and put 3 squirts of oil in the cylinder and re-test it and see if your numbers are any higher. Do this for each cylinder.


If you get a much higher number wet than dry (20 or more), then your loss is in the piston rings.

If you get a low number wet and dry, then you may have a problem with the valve seats. The next step would be to do a cylinder leak down test if you are interested in finding out which valve in which cylinder is leaking. Or remove the heads and get a valve job.

If you get a similar number wet and dry (about 5 psi) that is over 110-120 psi, then you should be ok.

 

[BillGrissom]

Doesn't matter. I have done a compression test with the throttle at idle then propped wide open (noticed mistake) and no difference in readings. By removing all spark plugs, the engine spins a bit faster since less load on the starter. Probably doesn't matter and is perhaps a bit cheating to fudge the readings upward from "normal cranking" condition. You must remove all the spark plugs anyway, so why not initially? Smart to put a rag over each hole so you don't drop anything down.

I should mention that you can get an excellent indication of cylinder sealing without a compression gage. Just slowly turn the engine over by hand and feel for 3 strong "air springs" per revolution (chalk mark crank pulley). If you hold at each and it takes >5 sec to leak down (should hear hissing), that cylinder has excellent seal (valve seats and piston rings). You can turn a slant over by pulling/pushing on the fan belt, and an easy test when you looking to buy a car. Just insure the key is out, or better to remove BATT- cable.

 

[Ruger64]

Thanks guys. I will do it in a bit here. With the plugs out.
I honestly feel my old carter Carb is just wore out.
Even the rebuilt made it run slightly better.
But it should not be having a slight damp issue between the top of the carb, and the mid section, where the gasket is supposed to seal it. I think I need a carb.
I will report back with a proper test tonight.
Hopefully, with good news...#-o

Also, I tried to adjust my timing, and it wasn't even near the timing tab. "The mark on the pulley", and I ran out of adjustment on the dizzy.
It is maxed on that little slot adjustment.
I think the pulley after years has slid on that rubber thing.
Because I was getting a reading using the dial on my timing light, to get it to TDC on the pulley, and my timing gun as showing while the mark was at TDC, the dial on my gun shows it at 32 degrees..."WoW"
But no bad hot starts, starts right up and runs great!!! For except the sputtering and roughness once in a while at low rpm's.
Or....another guess, if this is possible?
There were really crappy wires on the car when I got it, and I can tell someone was messing around with them.
I am wondering if he was "one rotation" off, when he put the wires on?